Acoustic liners and abradable liners are employed in front and rear fan casings of gas turbine engines. Acoustic liners are typically fabricated using a sandwich construction of a GRP (Glass Reinforced Plastic) or aluminium perforate outer skin, an aluminium honeycomb core, and a GRP backing skin for attachment to an inner surface of the casing. Abradable liners can have a similar construction, but typically have an abradable layer to form a track for the tips of the engine fan blades, and may have a non-metallic honeycomb core such as Nomex™ from DuPont.
Each liner may be formed in panel segments, the panels of a row of being butted together to form a complete ring around the inner surface of the fan casing. The panels may have compression moulded front and rear rails to provide structural integrity.
The panels can be attached to the fan casing using adhesive, for example using “vacuum bagging” to conform the panels to curvature of the casing while the adhesive cures, and to avoid entrapment of voids and volatile gases in the adhesive region. Another option is to bolt the panels to the casing.
A problem can arise, however, in that water may leak into the panels and degrade particularly the bonds formed between the outer skin and the honeycomb core.